This invention relates to a mirror, particularly a vehicle rear view mirror, comprising a mirror plate, a reflection layer and a scratch resistant protective layer.
In German Patent Document DE 38 25 164 A1, a mirror of this type is a heatable outside vehicle mirror which has a mirror plate to which a reflection layer is applied, and the surface facing the person looking into the mirror is formed by a glass pane. The mirror support can be heated via electric supply devices. In order to cause an areal current conduction within the mirror plate, contact strips, which are connected with the electric supply devices, are provided on the sides of the mirror support. It is a particular disadvantage that the glass pane has a relatively large thickness which requires a high energy supply when the mirror is heated before precipitations are dissolved on the mirror surface. The thickness of the glass pane cannot be reduced since, as a result of the different coefficients of thermal expansion, stress is caused in the glass pane which would destroy it if it had a thin construction or it would impair the optical characteristics of the mirror. In a variant, the reflection layer is applied to the mirror support and, instead of the glass pane, a scratch-resistant and weathering-resistant coating is provided on the reflection layer. This construction has the disadvantage that the thin scratch-resistant and weathering-resistant coating may cause interference phenomena of the impinging light in the viewing surface of the mirror which become noticeable as interfering color fringes in the mirror image.
It is an object of the invention to provide a mirror of the above-noted general type which permits the use of materials which have at least partially very different coefficients of thermal expansion for the layers applied to the mirror surface or for the mirror plate. Furthermore, also in the case of a heatable mirror, it is to be possible to use a thin glass layer as a scratch-resistant protective layer on the side of the mirror which faces the viewer of the mirror.
This object is achieved according to preferred embodiments of the invention by providing a mirror assembly comprising a mirror plate, a reflection layer on top of the mirror plate, and a scratch-resistant protective layer on top of the reflection layer, said protective layer facing a viewer of the mirror when in use, wherein an adhesive layer is disposed between the reflection layer and one of the mirror plate and the protective layer, said adhesive layer consisting of a silicone gel made of an addition cross-linking two-part silicone rubber.
It is particularly advantageous that the adhesive layer made of silicone gel, on the one hand, connects the two adjoining layers and, if these layers have very different coefficients of thermal expansion, prevents excessive tensions in these layers as a result of its elasticity. The adhesive layer made of a silicone gel therefore permits the use of materials which were previously not usable for the layer construction of a mirror, or of materials which, because of the thermal stress to be expected, had previously been used only with larger minimal thicknesses. For example, the silicone gel permits the use of a thin glass layer as a scratch-resistant and weathering-resistant protective layer on the mirror surface facing the viewer of the mirror. Whereas, in the case of the state of the art, the glass pane which forms a protective layer has a thickness of approximately 2 mm, the use of an adhesive layer made of silicone gel can significantly reduce the thickness of such a glass pane to, for example, approximately 0.04 mm. A glass pane of a reduced thickness used as the protective layer has the important advantage that a significantly lower energy supply is required for the heating of the mirror before precipitations evaporate on the surface of the mirror.
Also when another protective layer, such as a plastic coating or a lacquer coating, is used on the mirror side facing the viewer of the mirror, the individual layers may at least partially have very different coefficients of thermal expansion because, when the mirror is heated by the sun or by a heater, the adhesive layer made of a silicon gel will prevent excessive thermal stress in the individual layers or in the mirror support. This effect of the adhesive layer made of silicone gel, which balances thermal stress in mirror layers and/or in the mirror plate, exists independently of whether the silicone gel is arranged between the mirror plate and the reflection layer and/or between the reflection layer and the scratch-resistant protective layer. If the protective layer facing the viewer of the mirror is formed by a thin glass pane, this glass pane can be glued on in an areal fashion without any breakage of the glass under the occurring thermal stress or without any impairment of the optical characteristics of the mirror. The silicone gel is known per se and can be purchased, for example, as SilGel 612 from the firm Wacker-Chemie. This Sil-Gel 612 is an addition cross-linking two-part silicone rubber which vulcanizes in a gel-type manner. Up to now, this silicone gel has been found to be suitable for use as a transparent sealing of electric and electronic components, as seals for clean-room filters, as transparent gluing, for producing adhesive tapes, for embedding glass fibers and for the manufacturing of shock absorbers. The use of the silicon gel according to the invention for the layer construction of a mirror which permits a use of materials having very different coefficients of thermal expansion has not been known so far.